Current sensing circuits using so-called current sensing transistors (or “sense FETs”) have been commonly used for years. Such current sensing techniques may be especially useful when measuring the load current of power field effect transistors (power FETs), which are composed of a plurality of transistor cells as illustrated. Such power field effect transistors have a common drain region for all transistors cells composing the power transistor component. The common drain region is connected by one drain electrode arranged on the back-side of a wafer whereas the source region and the respective source electrodes are contacted on the front-side of the wafer and connected in parallel. The source electrode of a one transistor cell (referred to as “sense cell”) may be separately connected to tap a current signal that is representative of the load current flowing through the plurality of transistor cells of the load transistor. Of course a few transistor cells may be connected in parallel to form the sense transistor.
In circuit arrangement including a load transistor/sense transistor pair the sense current of the sense transistor is directly proportional to the load current of the load transistor whereby the factor of proportionality results from the ratio of the current conducting area of the load transistor and the current conducting area of the sense transistor which is (at least approximately) equivalent to the ratio of the number of transistor cells in the load transistor and, respectively, the sense transistor.
However, the differential amplifier responsively coupled to the sense transistor to draw the sense current of the sense transistor to draw the sense current from the sense transistor is usually employed to bias the sense transistor by a negative feedback. The negative feedback to bias the sense transistor is only possible as long as the flow directions of the load current and sense current through the load transistor and sense transistor is the same.
For instance, there are numerous applications switching high-current loads in high-side and low-side domains e.g. DC-DC converters, H-bridges, solid relays and the like. One of widely used approaches to measure a current flowing through the load transistor in the above use cases is to use a sense transistor with scaled down geometries as described above. Usually a negative feedback is employed to bias the sense transistor with a voltage or a current to equalize voltage drop at the sense transistor with the voltage drop at load transistor.
If there is a common terminal between load and sense transistor and they have the same direction of current flows the conventional implementations in the art can be employed for current measurements. In case the current though load transistor varies the flow direction (bi-directional) or it is not feasible to have same direction of current flows in the load transistor and sense transistor the conventional implementations in the art cannot be employed for current measurements.
Hence, there is a need for a current sensing circuits which can be used in case the load current and sense current through the load transistor and sense transistor have different directions.